The present invention is generally related to communication devices. More particularly, the present invention is related to a method for iterative demodulation and decoding for a system with coding and differential modulation and demodulation.
Digital radio systems employ channel coding and digital modulation of a carrier signal. Data to be transmitted include data representative of speech and control information. The data is encoded in a standardized format, with bits being placed in predetermined locations in the data stream and additional bits added for reliability. The encoded data stream is used to modulate a carrier signal, for example, using differential quadrature phase shift keying (DQPSK) and transmitted to a remote location. In a time division multiple access (TDMA) system, data is transmitted in a sequence of bursts.
A receiver which receives the transmitted signal performs essentially the reverse process. The modulated signal is demodulated and the encoded is decoded. Also, a reliability check is performed to determine that the data have been accurately received and decoded. If not, the transmission is rejected and a new burst is processed.
Previous receivers have used coherent demodulation for demodulating the signal. In coherent demodulation, the demodulator makes an estimate of the channel between the transmitter and the receiver and a hypothesized value of the received data is prepared. The actual received data are compared to the hypothesized data to determine reliability.
The major drawback of coherent demodulation is that it requires complicated mechanisms for tracking both the magnitude attenuation and the phase rotation induced by the channel. A coherent demodulator introduces substantial complexity into the receiver. Along with complexity, a coherent demodulator is relatively expensive to implement.
One alternative is differential demodulation, which is simpler and therefore cheaper to implement. However, differential demodulators perform significantly worse than coherent demodulators do.
Accordingly, there is a need for a demodulator which provides the performance advantages of a coherent demodulator without the complexity required for performing a channel estimate.